1. Field of the Invention
The present invention relates, in general, to a rotating pipe joint which permits two substantially axially aligned joined pipes to move axially, laterally and rotationally relative to one another and, in particular, to a rotating pipe joint which has an improved floating seal assembly which permits an amount of angular misalignment between the rotating and stationary pipes while still providing an acceptable seal.
2. Description of the Related Art
The present invention responds to the continuing need in the air heater industry for an effective seal assembly for sealing the fluid conveying duct to the sootblower of a Rothemuhle air heater while permitting the two joined pipes to rotate relative to one another. The invention is applicable wherever a rotating pipe joint is required in a hostile environment and a less than complete seal is satisfactory.
In a Rothemuhle air heater, the heat transfer surfaces comprise a stationary stator, which is too large to be desirably rotatable. A hood above the stator and a hood below the stator rotate in unison, connected by a single, axial, hollow, rotating, centering post. Aligned rotating portions of the upper and lower hoods provide one passageway for gas through the air heater heat transfer surfaces. The remaining hood portions provide another passageway. The rotating hood portions may conduct flue gas or air, depending on the needs of a particular installation. The remaining hood portions conduct whichever fluid the rotating hood portion does not conduct. As the hoods rotate, the heat transfer surfaces are alternately heated by flue gas and cooled by incoming combustion air.
The flue gas deposits deleterious materials, such as carbon, fly ash, etc., onto the heat transfer surfaces, reducing their ability to transfer heat. Periodic removal of these materials from the heat transfer surfaces is required to obtain efficient operation and long life from the air heater. Sootblowers have long been used for cleaning air heaters while they are in service. In a Rothemuhle air heater, the sootblower is attached to the upper or lower rotating hood so that it rotates in unison with the rotating hood. In some cases a separate sootblower is attached to each rotating hood. The sootblower orifices are aligned to blow high pressure fluid, customarily air or dry steam, into the heat transfer surfaces of the air heater to dislodge the accumulated deleterious materials. Typically a single sootblower is provided on the "cold" side of the air heater, but in some cases a sootblower may also be provided on the "hot" side as well. The air heater hoods rotate in the range of about 1 to about 2 rpm.
A rotating centering post provides a common drive for the upper and lower hoods, serves as a pipe which conveys the fluid to the sootblower, and is connected to a stationary inlet pipe; as such, the rotating joint between them must accommodate this rotation and provide an acceptable seal. The joint must survive in the hostile environment of the air heater where temperatures of 650.degree. F. or more are typically encountered. In addition, the rotating joint is inaccessible while the air heater is in service, and is difficult to reach in any event.
The air heater is most conveniently removed from service for maintenance during regularly scheduled boiler maintenance, typically scheduled once a year. If the air heater must be removed from service while the boiler is operating, as would be required if the sootblower fails because the sliding seal or the expansion joint has failed, several undesirable effects occur. The air heater must be allowed to cool, which increases unit downtime. More importantly, the boiler itself must usually be removed from service if the air heater is stopped for maintenance. Skilled workers must enter the air heater, discern the cause of the sootblower failure, and replace the defective parts. The sliding seal itself could cost several thousand dollars, and the additional costs incurred in an unscheduled shutdown of the air heater and boiler make failure of the sliding seal especially expensive and undesirable.
One solution to the above-identified problems is shown in U.S. Pat. No. 4,659,114 to Gerber--ROTATING PIPE JOINT HAVING A FLOATING SEAL, assigned to The Babcock & Wilcox Company. A sketch of this floating seal assembly is provided in FIG. 1 of the present disclosure. As set forth therein, the rotating pipe joint 10 includes a stationary inlet pipe 12 and slide pipe 14 slidably and rotatably connected via a housing 16 to a rotating centering post 18. The housing 16 is defined by a thrust plate 20, spacer plate 22, and a flange 24. The flange 24 is fixedly connected to the rotating centering post 18 by means of a support pipe 26, bolts 28, and a base 30. The housing 16 encloses a floating seal 32 comprising a collar 34, preferably having a plurality of compression rings 36 which are inwardly biased against the slide pipe 14 to provide a seal between the slide pipe 14 and the collar 34. Rotational and axial movement, the latter due to thermal expansion, are accommodated between the slide pipe 14 and the compression rings 36. A limited amount of lateral movement, in the range of 3/4" to 11/2" from the vertical centerlines of the two pipes 12 and 18, is accommodated by an aperture 38 in the thrust plate 20 and by virtue of the fact that the outside diameter of the collar 34 is smaller than the inside diameter of the housing 16, which defines an inner chamber 40. Gaps 42 and 44 vary in size, depending upon the degree of pressurization of the chamber 40. In operation, the upper surface of collar 34 is thrust against thrust plate 20, eliminating gap 42 and sealing the joint. At this point, most relative rotation occurs between the collar 34 and the slide pipe 14, since the collar 34 tends to stick to the thrust plate 20.
FIGS. 2 and 3 show various known constructions of the collar 34 shown in FIG. 1. In FIG. 2, for example, collar 34 is provided with the aforementioned compression rings 36 seated in grooves 46, gaps 48 occurring at the ends of the compression rings 36 being staggered to further reduce leakage. In FIG. 3, another known construction for the collar is shown, designated therein as 34', wherein aperture 50 is tapered about 5.degree., although tapers of 3.degree. to 15.degree. are acceptable, to accommodate toggling of the slide pipe 14 within the collar 34' without adversely affecting the seal.
While the construction of U.S. Pat. No. 4,659,114 represents an advance in the art, room for further improvement exists. Accordingly, it has become desirable to develop a rotating pipe joint which has an improved floating seal assembly which permits an amount of angular misalignment between the rotating and stationary pipes while still providing an acceptable seal.